Ionomer resins have been widely used as the golf ball cover material. Since ionomer resins can impart improved play characteristics such as durability and resilience, they are advantageously used as the base resin of golf ball cover materials.
The ionomer resins are ionic copolymers of an olefin such as ethylene and an unsaturated carboxylic acid such as methacrylic acid or maleic acid wherein some acidic groups are neutralized with a metal ion such as sodium, lithium, zinc or magnesium ion.
The use of the ionomer resins as the golf ball cover material, however, has drawbacks including a hard hitting feel and the lack of flexibility necessary to impart a sufficient spin to control the ball in flight. Since the ionomer resins are relatively hard, the ball cannot be fully impressed against the club head surface upon impact so that the ball may receive a reduced spin rate. If a harder ionomer resin is used as the cover material, the feel of the ball accordingly becomes very hard.
It is thus desirable to have a golf ball having good spin properties and a pleasant feel in which experienced low-handicap golfers and professional golfers are interested. Great efforts have been made to this end.
Specifically, in order to overcome the drawback of ionomer resin that the feel of the ball when hit is hard and to further improve the resilience characteristic of ionomer resin, a number of golf ball cover compositions have been proposed. For example, JP-A 5-3931 discloses a cover composition in which a soft ionomer resin obtained by neutralizing an olefin-acrylic acid-(meth)acrylate ternary copolymer is blended with a high hardness ionomer resin for the purpose of improving the feel of the ball when hit. Using as the cover material a mixture of a hard ionomer resin and a soft ternary ionomer resin containing a (meth)acrylate as one monomer, the ball is increased in distance at no sacrifice of durability.
The cover material comprising in simple admixture the hard ionomer resin and the soft ternary ionomer resin maintains approximately equivalent resilience as compared with a single ionomer resin of the same hardness, but fails to achieve a significant improvement in resilience.
U.S. Pat. Nos. 5,312,857 and 5,306,760 disclose cover compositions in which high levels of metal soaps are added to ionomer resins for the purpose of improving the cost and resilience of ionomer cover material. Allegedly, these compositions are cost effective and increased in resilience (or coefficient of restitution) as compared with prior art ionomer cover materials.
The cover compositions of these patents, however, have very low paint coatability. When the cover of a two-piece ball or the outer cover layer of a multi-piece ball is made of such a cover composition and coated with paint, the cover is often repellent to the paint coating. Even when the paint coating uniformly adheres to the cover, the coating can be readily peeled off by impact. As compared with ionomer resin covers of the same hardness, an improvement in resilience is made by the addition of metal soaps to the same extent or a slightly greater extent, but not to a greater extent. A particular type of metal soap added can significantly detract from moldability and resilience. From these points of view, the cover compositions of the above patents are unacceptable for practical use.